A non-volatile memory is a computer memory device which can retain the data stored even when no power is applied. Examples of non-volatile memories widely used today are read only memories, flash memories, and optical and magnetic disc drives.
Flash memories are widely used in memory cards and USB drives to store data and to transfer data between a computer and other digital devices such as cameras and mobile phones. Flash memories are implemented using floating gate transistors, which generally comprise a MOS transistor capacitively coupled to a number of secondary gates.
Because the floating gate is electrically isolated from the secondary gates, any charge trapped on the floating gate is retained for a long period of time without a power supply being required. By applying voltages to the source, drain and secondary gates, the charge stored on the floating gate can be altered. The charge trapping region in the floating gate can be fabricated using a number of different materials, for example polysilicon, silicon nitride or a nanocrystal charge trap structure.
A problem with this technology is a lack of high temperature data retention. In the latest devices, at high temperatures, electrons stored either in poly-silicon floating gate, nano-crystal or silicon nitride trapping layer are thermally exited and easily escape from the storage medium. As a result, data retention is severely compromised and so such devices do not operate reliably.
An alternative implementation of a non-volatile memory uses micro electromechanical system (MEMS) technology.
US 2007/0086237 describes a non-volatile memory using a shape memory alloy which returns to a predetermined shape when heated above a certain temperature, referred to as the Martensite transition temperature. The shape memory alloy is used to implement a transistor a gate for a field effect transistor or a mechanical switch. The shape memory alloy is actuable to different positions by applying an electrostatic field or heat, and dependent on the position, the memory stores either a logic 1 or logic 0.
US 2008/0144364 describes an electromechanical memory device with a cantilever electrode formed from a shape memory alloy arranged between two word lines. Applying an electrostatic force causes the cantilever electrodes to deform sufficiently to contact the word lines. Once the cantilever electrode is in contact with the word lines, contact can be continuously maintained by Van der Waals's force of attraction between electrode and the word line. The cantilever electrode is released by passing a current through the electrode causing the shape memory alloy to heat above the Martensite transition temperature and to return to its original shape, breaking the contact between the electrode and the word line.
WO 94/27308 describes a bistable memory element with a base contact and a bridging contact. The bridging contact is deflected to be in contact with the base contact by applying an electrostatic force in one direction and deflected away from the base contact by applying an electrostatic force in the opposite direction.